Current interruption devices generally comprise electromagnetic circuit breakers for protecting electrical loads from electrical power overloads or surges. Current interruption devices typically employ mechanical contacts which separate in response to an overload condition in order to separate the load from the power source and thus protect the load from the potentially damaging overload condition. The use of electromagnetic relays employing movable contacts has been found to be superior to solid state switches insofar as the contact resistance of the former is substantially less objectionable than the conduction resistance of solid state switches. Electromagnetic relays typically waste less energy and generate less heat than comparably rated solid state switches, at least for circuit interruption devices.
A drawback with movable contact relays is that the contact opening and closing transition time is relatively slow relative to the turning on and off time of the solid state switches. The relatively slow contact transition time and the relatively large voltage difference across open contacts often results in undesirable arcing across the contacts during contact transition.
U.S. Pat. No. 4,700,256 to Howell is directed to circuitry for eliminating arcing across switched contacts. The device employs a parallel combination of mechanical and solid state switches. When the mechanical switch is opened, a voltage difference increase or build-up between the contacts causes electronic circuitry to switch on the solid state switch to temporarily conduct in order to slow down the voltage increase, thereby minimizing arcing. Unfortunately, because the solid state switch is turned on in response to voltage build-up across the contacts, there is a slight delay in turning on the solid state switch and thus some arcing may nonetheless occur which can lead to damage of sensitive electronic components used either in the current interrupter itself or in close proximity thereto.
U.S. Pat. No. 5,164,872 to Howell is directed to a load commutation circuit for arcless interruption of ac current to a load. The device employs a primary current path through a pair of solid state switches which are turned off in response to an overload condition. The current once flowing through the switches is next shunted to and dissipated from a current diverter circuit. Because no current flows through the primary path after the current is shunted, a mechanical switch interposed along the primary current path can be opened without arcing across the switch contacts. A drawback with the above approach is that the primary current path is through the relatively high resistance solid state switches. These solid state switches which may require high current ratings may thus tend to be expensive. Furthermore, the relatively high resistance of the solid state switches will waste electricity in the form of heat generation which must be adequately dissipated. In order to adequately dissipate the heat, the switches tend to be widely spaced from adjoining components, thereby resulting in a relatively large device.
In view of the foregoing, it is an object of the present invention to overcome the drawbacks and disadvantages of prior art current interrupters.